Do you mean the proposal for applying to do a PhD or the formal proposal you have to submit to the university for things like ethical approval after you've started? I think my initial one was 3 pages (that was then the ESRC maximum) and the formal approval was on a form so it had quite tight space constraints for each section.
My instution set a word limit, which I went over IIRC (but because it's interdisciplinary like yours, Dan, I got away with it). I think mine was meant to be 3,000 words.